Senator Robert F. Kennedy was assassinated during the filming of this episode, on June 6, 1968. France Nuyen (Elaan) and her then-husband, Robert Culp
, were close friends of his and campaigned for him a great deal. Nuyen was, understandably, extremely distraught.
Scotty (James Doohan) would later refer to the events of this episode, particulary Elaan's dissatisfaction with the quarters she was assigned, in the Star Trek: The Next Generation
John Meredyth Lucas both writes and directs this episode: the only person to do so in the original series.
A scene with Spock playing the Vulcan lyre and Kirk pumping it into Elaan's quarters in an attempt to calm her down was filmed but cut. The sequence appears in James Blish's adaptation of the episode.
This is the first episode in production order to show an actual Klingon ship, created by Matt Jeffries. Previous episodes had either kept them off-screen, or shown them as indistinct blobs. "Day of the Dove" was produced later but aired earlier, so footage from this episode ended up recycled there.
Elaan: You rule this ship?
Spock: I am the First Officer. This is Captain James Kirk.
Kirk: Ambassador, I'd like to speak to you.
Elaan: You have not been dismissed.
Kirk: May I have your permission to go?
Elaan: You are now dismissed.
Kirk: Why, thank you.
Scotty: Captain... you'll not be using the warp drive? All the way on impulse? That'll take a great deal of time.
Kirk: You in a hurry, Mr. Scott?
Spock: Captain, the Dohlman is dissatisfied with the quarters provided.
Uhura: What's the matter with them, Mr. Spock?
Spock: I do not know, but all the Elasians seem most irrational.
Kirk: If that's the only way you can get gratification, I'll arrange to have the whole room filled from floor to ceiling with breakable objects.
Elaan: I will not be humiliated!
Kirk: Then act in a civilized fashion.
Elaan: I did not give you permission to leave!
Kirk: I didn't ask for any.
Kirk: Ambassador! This is supposed to be a peaceful mission.
Petri: There cannot be peace between us. We have deluded ourselves. Captain, when I am near them, I do not want peace. I want to kill them!
Elaan: We are interested in how the ship is used in combat, not in what drives it. Engines are for mechanics and menials.
Scotty: Menials? Without engines, there'd be no fighting or...!
Elaan: We have granted your crew the permission not to kneel in our presence. What else do you want?
Kirk: Courtesy. Remember?
Elaan: Courtesy is not for inferiors.
Kirk: Courtesy is for everyone around here, and you'll find you won't be able to exist on Troyius without it. Mr. Scott, our chief engineer, has shown you his engineering department. That's a courtesy. You respond by saying "thank you."
Kirk: It's been my experience that the prejudices people feel about each other disappear when they get to know each other.
Elaan: It's not my experience.
Elaan: Nobody speaks to me that way.
Kirk: That's another one of your problems. Nobody's told you that you're an uncivilized savage, a vicious child in a woman's body, an arrogant monster! (she slaps him, Kirk slaps her back) That's no way to treat someone who's telling you the truth. (she throws a knife) Tomorrow's lesson will be on courtesy.
Spock: Captain, your analysis of the situation was flawless, anticipating that she would deny you admittance. However, the logic by which you arrived at your conclusion escapes me.
Kirk: Mr. Spock, the women on your planet are logical. That's the only planet in this galaxy that could make that claim.
Elaan: You dare touch a member of the royal family.
Kirk: Only in self-defense.
Elaan: You are warned, Captain, never to touch me again.
Kirk: If I touch you again, Your Glory, it'll be to administer an ancient Earth custom called a "spanking," a form of punishment administered to spoiled brats.
Elaan: Captain, that ancient Earth custom called "spanking"... what is it?
Kirk: It's, um... it's, uh... we'll talk about it later.
Kirk: Scotty, he didn't come into Engineering just to use the transmitter. I want you to check every relay you've got.
Scotty: Captain, you must realize the time...
Kirk: Don't waste the time telling me about it. Get to it.
McCoy: Spock, I've isolated the biochemical substance of the Elasian tears. It's a kind of infection, and I think I found an antidote.
Spock: Too late, Doctor. The captain has found his own antidote.
McCoy: Are you out of your Vulcan mind? Do you know how long...
Spock: The antidote to a woman of Elas is a starship. The Enterprise infected the captain before the Dohlman.
Kirk: Mr. Sulu, prepare to take us out of orbit.
McCoy: Well, I doubt seriously if there's any kind of an antidote for the Enterprise.
Spock: In this particular instance, Doctor, I agree with you.
In the opening sequence, Scotty is standing by the room to the transporter room, but when they cut to the closeup shot of the hands on the transporter controls, they're Scotty's.
Although a very short period of time passes between Kirk taking Elaan to Sickbay, returning back to the bridge, her having a conversation with Petri, and her returning to the bridge to be with her beloved Kirk. However, despite the crisis situation and the threat of imminent death, she's taken the time to change from her orange dress to a blue one.
When the Klingons close in for the first time after the sabotage is discovered, a long shot from behind Kirk shows Hadley at the navigation station. Another a shot a few minutes later when Kirk says "Arm photon torpedoes" again features Hadley , then cuts almost immediately to Chekov.